This invention relates to the production of multiple layer sheeting. More particularly, the present invention relates to the extrusion of plastic sheeting having a thin surface veneer of a second plastic material on at least one side thereof and to the compositions thereby produced.
Due to the particular properties of the many plastics, certain plastics are required for certain specific end applications and other plastics cannot be used in such applications. In many instances, due to this requirement of certain specific properties for particular applications, the cost of the plastic material is rather high as compared to similar plastics having somewhat different properties.
Within recent years, considerable effort has been made to find means of reducing the cost of these specialized plastics without an attendant loss of the properties which have made the plastics suitable for the particular uses in which they are employed. Among the means explored have been lamination or coextrusion together of different plastics to produce a composite. These techniques have in common the bringing together in layered form, sheets of two or more different plastic materials. In such manner, composite sheets may be produced having surface and other characteristics of the desired speciality plastic and yet by using a much cheaper substrate than the superstrate speciality plastic, the costs of the plastic sheet can be materially lowered.
In lamination, two or more previously extruded sheets of different plastics are brought together under pressure and temperature conditions or in the presence of adhesives in order to obtain adherence of the different plastic sheets to one another. This technique of forming composite sheets has a disadvantage of little flexibility in sheet thicknesses, requirement of extra extrusion equipment such as two or more regular extruders with attendant dies, rollers, sheet line, etc. Additionally, without the use of adhesives, adherence of the layers to one another is frequently unsatisfactory.
Coextrusion offers the least expensive means of preparing layered composite sheets of different plastics. Within coextrusion, two different techniques are most often employed. In one of these techniques, two or more plastic sheets are extruded from separate extruders through separate sheet dies into contact with one another while still hot and then passed through a single set of rollers or another extrusion die and down a single sheet line. Employing this technique, equipment requirements are still relatively large and there is still little flexibility and adherence is still quite frequently a problem with many plastics. The other coextrusion technique employs an adaptor or other means of bringing two or more different plastic materials from two or more extruders into contact with one another prior to their passage through an extrusion die. Generally, the known coextrusion processes using this technique have employed some form of encapsulation technique wherein one stream of thermoplastic material, typically the volumetrically smaller stream, is completely surrounded, e.g., coaxially, by a second stream of a different plastic material prior to passing the entire composite stream through an extrusion die. Alternatively, such incapsulations may be effected in the cavity portion of the extrusion die itself. In either instance, however, the resulting sheet product is characterized by an inner layer of one type of plastic material sandwiched between or encapsulated by two exterior layers of a second plastic material.
One of the major problems in coextrusion wherein the plastics to be coextruded are brought into contact with one another prior to passing through a die, has been the determination of conditions and means for bringing together components in a manner such as to produce upon extrusion through a sheet die, uniform layers while still obtaining the desired adherence of the plastic materials to one another. Additionally, prior coextrusion processes of this type have had limitations as to the relative thicknesses of the layers, requiring the minor or superstrate layer to represent at least 25% of the total volume of the composition.
It is now an object of the present invention to provide a new and improved process for coextrusion of a multiple layered sheeting.
Another object of the present invention is to provide a new and improved process for the coextrusion of two or more plastic materials into a composite sheeting whereby one or more of the layers may be very thin, representing less than 25 volume percent of the total composite.
A further object of the present invention is to provide a method of coextruding polystyrene and polycarbonates.
A remaining object of the present invention is to provide a new and novel plastic composition having at least two layers of thermoplastic material, at least two of said plastic materials being polystyrene and polycarbonate and being in contact with one another.
Additional objects will become apparent from the following description of the invention herein disclosed.